It is often necessary to inspect structural components of large objects, such as aircraft, maritime vessels, automobiles, and other large investment assemblies, for defects and damage. Other structural objects that may require inspection include petrochemical facilities, power generation facilities, nuclear facilities, water treatment facilities, and the like.
Inspection of such structures and facilities by partial or complete disassembly of the structures to visually inspect internal components of interest may be impracticable because it is too time-consuming, and expensive. In some instances it may not be possible to inspect the desired component without partially destroying the component of interest. Further, transportation of such structures and components of large object or facilities to an inspection location may be difficult, expensive and in some instances, impossible.
A technique for inspecting such components is the use of x-ray imaging. Inspection by x-ray imaging requires an x-ray transmitter that transmits x-rays sufficiently powerful to pass through the object of interest and its surrounding components to be detected by a film or other detection means. Such devices are large, cumbersome and relatively expensive.
X-ray backscattering systems provide an inspection process in which x-rays are reflected from the object or component of interest and recorded by a detector or detectors. X-ray backscattering systems do not need to be powerful enough to transmit x-rays entirely through the component of interest and its surrounding components. Rather, partial penetration is all that is required. X-ray backscatter inspection systems are smaller, more portable, and less expensive than traditional x-ray imaging systems.
A problem with x-ray backscattering systems is that the reference standard used for calibrating or adjusting traditional through-transmission film or digital x-rays does not work for x-ray backscatter systems. Scattered x-rays off of the standards or gauges used for traditional through-transmission x-ray inspection do not provide relevant spatial frequency information because x-rays scattered differently (i.e., with respect to intensity and angle) from different types of materials. Accordingly, there is a need for a method and system for quantifying x-ray backscatter system performance.
Further, in order to image very small features or detect small cracks such as stress corrosion cracks, the size of the openings in the aperture of an x-ray backscatter device must be very small, on the order of 0.25 millimeters (mm) or 0.5 mm in diameter. However, with such small apertures, the amount of x-ray flux that is emitted from the x-ray tube is very low, which requires a relatively slow x-ray tube scan speed and a relatively small scan area for each raster scan, so that many more raster scans are required for a given inspection area. Often, x-ray tubes with relatively larger apertures may be used for general scanning because they permit higher x-ray flux, which enables a relatively larger area to be scanned for each pass, and at a faster scan rate. However, switching between large and small apertures takes time and requires multiple sets of apertures of different sizes. Another disadvantage is that smaller apertures are more difficult and expensive to fabricate than larger apertures. The dimensions of small apertures make reproducibility of apertures difficult and require software normalization to achieve consistent flux intensities from the x-ray tube.